FIGS. 1 and 2 are diagrams for describing a radio communications scheme using UWB (Ultra Wide Band) technology, and are extracted from a document numbered IEEE802.15-03-267r5 which has been submitted to the IEEE802 Committee.
As illustrated in FIG. 1, in a multi-band OFDM (Orthogonal Frequency Divisional Multiplexing) scheme which is one type of UWB proposed in the document, a frequency band from 3.1 to 10.6 GHz freed as a UWB consumer available band is divided into 14 sub-bands, some of which are used in combination. A mode-1 device, which is one of them, utilizes three sub-bands Band#1-Band#3 on the lower frequency side shown in FIG. 2. Specifically, the mode-1 device transmits OFDM symbols while switching the three sub-bands every 312.5 ns, i.e., while hopping the frequency. Upon the frequency hopping, the frequency is switched in 9.5 ns.
A transceiver which employs such fast frequency hopping requires a local signal generator which is capable of switching from one frequency to another at high speeds. Frequency hopping within 9.5 ns cannot be carried out by a system which utilizes a PLL frequency synthesizer capable of fast frequency locking, which is used in a conventional frequency hopping system. The reason for this is attributable to the inability to realize a voltage controlled oscillator which can vary the frequency over such a wide band, and has phase-noise characteristics which endure radio communication applications. Also, for completing frequency switching within 9.5 ns, a plurality of phase comparison operations are required in the period of 9.5 ns, so that the phase comparison frequency is at least on the order of GHz, thus experiencing difficulty in fully satisfying phase-noise characteristics, power consumption, and cost required for mobile radio communications.
FIG. 3 is a diagram illustrating a conventional local signal generator which supports fast hopping.
As illustrated in FIG. 3, this related art example comprises an oscillation source comprised of voltage controlled oscillator 81 and PLL 82 for generating a frequency of 4224 MHz, which is divided by 1/8 frequency divider 83 to generate a signal at 528 MHz, which is again divided by 1/2 frequency divider 84 to generate a signal at 264 MHz. Then, the signal at 264 MHz and signal at 528 MHz are applied to single side-band mixer (SSB mixer, SSB) 85 to generate a signal at 729 MHz which is the sum of the two frequencies. One signal is selected by selector 86 from among the signals at 264 MHz and the signal at 792 MHz, and supplied to another single side-band mixer 87. This second single side-band mixer 87 generates a signal at 4488 MHz which is the sum of the frequencies 4224 MHz and 264 MHz; a signal at 3960 MHz which is the difference between 4224 MHz and 264 MHz; and a signal at 3432 MHz which is the difference between 4224 MHz and 792 MHz, respectively.
In this connection, the single band mixer switches a differential frequency and a sum frequency for delivery in the following manner. First, sin and cos are provided for a component at frequency f1 and a component at frequency f2, respectively. Exemplary signal processing for generating the sum frequency may involve the following method:sin(2×π×f1×t)×cos(2×π×f2×t)+cos(2×π×f1×t)×sin(2×π×f2×t)=sin {2×π×(f1+f2)×t}
On the other hand, the differential frequency may be calculated in the following manner:sin(2×π×f1×t)×cos(2×π×f2×t)−cos(2×π×f1×t)×sin(2×π×f2×t)=sin {2×π×(f1−f2)×t}
When all circuits comprise differential circuits, the sum frequency and differential frequency can be selectively provided, for example, by allowing switching between a negative and a positive sign for sin(2×π×f2×t) through the switching unit.
The related art example illustrated in FIG. 3, however, has problems as listed below.
Even while generating the sum frequency of 4224 MHz and 264 MHz, a differential frequency component therebetween, which is an undesired wave, is in actuality parasitically generated. Its magnitude depends on the image rejection ratio of the single side-band mixer. Taking into account variations associated with device fabrication and the like, the image rejection ratio generally takes a value of 30 dB more or less. Since the differential frequency corresponds to the carrier of Band#2 in FIG. 2, this differential frequency modulated with a base band signal is radiated as is from an antenna, when assuming a transmission system. Therefore, the rejection ratio of 30 dB cannot be said to be practically sufficient in some cases, so that a larger rejection is required as the case may be.
Likewise, even while generating the differential frequency between 4224 MHz and 264 MHz, a sum frequency component thereof, which is an undesired wave, is in actuality parasitically generated. This exerts the same influence as the aforementioned scenario in which the sum frequency is a desired wave and the differential frequency is an undesired wave, and a larger rejection is required as the case may be.
Also, when 264 MHz is generated by a frequency divider, the resulting waveform is generally rectangular. Therefore, a signal at 264 MHz includes 792 MHz which is a third harmonic thereof. In the course of generation of the sum frequency of 4224 MHz and 264 MHz or the differential frequency therebetween, a differential frequency between 4224 MHz and 792 MHz, caused by the third harmonic, is parasitically generated as well. Since this undesired wave component corresponds to the carrier of Band#1 in FIG. 2, this differential frequency modulated with a base band signal is radiated as it is from an antenna, when assuming a transmission system. In regard to the magnitude, the magnitude at 792 MHz is lower than the magnitude at 264 MHz by 9.4 dB, supposing that it is a completely rectangular wave. Consequently, the magnitude of the differential frequency between 4224 MHz and 792 MHz is lower by 9.4 dB with respect to the desired wave. This rejection ratio of 9.4 dB is not at all sufficient.
When single side-band mixer 85 mixes 264 MHz with 528 MHz, a generated 792 MHz component can vary in amplitude due to subtle fluctuations in frequency of the input signals, due to the fact that the third harmonic of 264 MHz and output wave are at the same frequency. This is because variations in amplitude are reflected as they are to variations in amplitude of a local signal, thus giving rise to inconveniences.
The single side-band mixer is a device which has a large chip area and consumes a large amount of power. The provision of two such devices directly results in an increase in chip area and an increase in current consumption.
As described above, the conventional PLL frequency synthesizer technology encounters difficulties in providing a local signal generator which is capable of switching frequencies at high speeds while satisfying the phase-noise characteristics, power consumption, and cost required for mobile radio communications.
Also, the related art example illustrated in FIG. 3 has a problem that a variety of undesired waves are generated, and the undesired waves modulated with a base band signal are radiated as they are from an antenna. Otherwise, another problem arises in which a local signal varies in amplitude.